4.2 Ionotropic Gelation
Ionotropic gelation is a simple microencapsulation method and can be used to
enhance the shelf life and controlled release of the bioactive compounds [49, 50].
Typically, ionotropic gelation is performed with alginate and makes use of the
gelation of this hydrocolloid in the presence of divalent metal ions. A major step in
ionotropic gelation is the mixing of the wall material and core material together. For
example, alginate solution is mixed with oil in water emulsion for about one hour
and then the mixture is added drop-wise into a solution containing calcium ions,
whereby gelation occurs through a reaction between alginate and the ions in
solution. The hardened capsules are subsequently collected and dried. The mechanism of gelation includes the addition of ions, such as calcium or sodium chloride,
which shield electrostatic charges on the surface of proteins and allow molecules to
move close together. As a result, molecules can aggregate due to the absence of
electrostatic repulsion and gelation can occur.
The major advantages of ionotropic gelation are that capsules are easily
prepared in aqueous solutions and at room temperature, enabling the method to
be used for heat-labile materials. The drawback of this method includes very low
payload, unacceptable cost-in-use, large capsules size, limited range of wall
materials and undesirable amounts of substantial polymers incorporated into the
foods. In order to improve this technique, hydrophobically modified starch,
epimerized alginate and polyelectrolyte-based multilayer microcapsules can be
used to increase the payload.
4.3 Coacervation
Coacervation is the process by which colloidal particles are separated from solution
and deposited around a core material. Coacervation is typically used for the encapsulation of flavours, lipid-soluble vitamins and phytochemicals and certain
enzymes. Simple coacervation involves the use of only one hydrocolloid, whereas
complex coacervation utilizes two or more polymers. The coacervation process
consists of three principle steps: phase separation, deposition and solidification. In
the initial step, the coating material, usually consisting of one or more polymers,
undergoes a phase separation to form a coacervate. Core materials are suspended or
emulsified in the same reaction media and, as particles coalesce, a decrease in
surface area occurs. Decreased surface area of the coating particles prompts a
decrease in total free interfacial energy of the system, which in turn favours coacervate nuclei adsorption to the core material surface. As a result, a uniform layer of
coacervate forms around core particles. In the final step, solidification of the coating
material is achieved by crosslinking using chemical, thermal or enzymatic methods.
Microcapsules are then collected by filtration or centrifugation before being dried.
Spraying a chitosan solution into sodium hydroxide, NaOH–methanol or
ethanediamine alkaline solutions using compressed air results in coacervated
droplets, forming the nanoparticles [51]. Separation and purification of the particles
Biopolymeric Micro- and Nanoparticles: Preparation, Characterization and. . .
281
Ionotropic gelation is a simple microencapsulation method and can be used to
enhance the shelf life and controlled release of the bioactive compounds [49, 50].
Typically, ionotropic gelation is performed with alginate and makes use of the
gelation of this hydrocolloid in the presence of divalent metal ions. A major step in
ionotropic gelation is the mixing of the wall material and core material together. For
example, alginate solution is mixed with oil in water emulsion for about one hour
and then the mixture is added drop-wise into a solution containing calcium ions,
whereby gelation occurs through a reaction between alginate and the ions in
solution. The hardened capsules are subsequently collected and dried. The mechanism of gelation includes the addition of ions, such as calcium or sodium chloride,
which shield electrostatic charges on the surface of proteins and allow molecules to
move close together. As a result, molecules can aggregate due to the absence of
electrostatic repulsion and gelation can occur.
The major advantages of ionotropic gelation are that capsules are easily
prepared in aqueous solutions and at room temperature, enabling the method to
be used for heat-labile materials. The drawback of this method includes very low
payload, unacceptable cost-in-use, large capsules size, limited range of wall
materials and undesirable amounts of substantial polymers incorporated into the
foods. In order to improve this technique, hydrophobically modified starch,
epimerized alginate and polyelectrolyte-based multilayer microcapsules can be
used to increase the payload.
4.3 Coacervation
Coacervation is the process by which colloidal particles are separated from solution
and deposited around a core material. Coacervation is typically used for the encapsulation of flavours, lipid-soluble vitamins and phytochemicals and certain
enzymes. Simple coacervation involves the use of only one hydrocolloid, whereas
complex coacervation utilizes two or more polymers. The coacervation process
consists of three principle steps: phase separation, deposition and solidification. In
the initial step, the coating material, usually consisting of one or more polymers,
undergoes a phase separation to form a coacervate. Core materials are suspended or
emulsified in the same reaction media and, as particles coalesce, a decrease in
surface area occurs. Decreased surface area of the coating particles prompts a
decrease in total free interfacial energy of the system, which in turn favours coacervate nuclei adsorption to the core material surface. As a result, a uniform layer of
coacervate forms around core particles. In the final step, solidification of the coating
material is achieved by crosslinking using chemical, thermal or enzymatic methods.
Microcapsules are then collected by filtration or centrifugation before being dried.
Spraying a chitosan solution into sodium hydroxide, NaOH–methanol or
ethanediamine alkaline solutions using compressed air results in coacervated
droplets, forming the nanoparticles [51]. Separation and purification of the particles
Biopolymeric Micro- and Nanoparticles: Preparation, Characterization and. . .
281